Elemental phosphorus is produced by smelting a mixture of phosphate ore, reducing carbon, and silica in a submergedarc electric furnace. The ore reacts with the reducing carbon at high temperature and in this reaction the combined phosphorus is reduced to the element. Some of the metal oxides present in the furnace feed materials are reduced to metallic elements. Iron oxide is a constituent in the furnace feed materials and this oxide is reduced to elemental iron which combines with elemental phosphorus to form a mixture of iron phosphides called ferrophosphorus. Other metals alloy with the ferrophosphorus. The ferrophosphorus is tapped from the furnace as a molten material.
Phosphorus furnace feed materials contain metal oxides which are not reduced by carbon to form the metal elements. The metal oxides combine to form a slag which is tapped from the furnace as a molten material. Both slag and ferrophosphorus are byproducts.
Hydrogen compounds react with reducing carbon in the furnace to form hydrogen gas. The hydrogen compounds are hydrocarbons which may be constituents of the reducing carbon. Furnace feed materials may contain moisture or combined water and this compound is reduced to form hydrogen gas. The hydrogen mixes with other gases volatilized from the furnace.
Carbon in the reducing carbon oxidizes to form carbon monoxide when the various compounds in the furnace are reduced. The carbon monoxide mixes with the other gases volatilized from the furnace. Most of the carbon monoxide is derived from the reduction of combined phosphorus in phosphate ore and only a small proportion is formed by the reduction of metal oxides and hydrogen compounds. Most of the elemental phosphorus formed by the reduction of phosphate ore is discharged from the furnace as a gas. Therefore, gases from the furnace are composed mainly of carbon monoxide and phosphorus. But the furnace gas contains other materials. Sources of other materials in the furnace gas mixture are given below.
1. Reducing carbons fed to the phosphorus furnace contain volatile constituents, and some of these constituents are volatilized without being reduced. Both saturated and unsaturated hydrocarbons are volatilized. Some reducing carbons, such as coals, contain combined water which may be reduced in the furnace to form hydrogen. PA0 2. Phosphate ore contains fluorine, and part of the fluorine is volatilized when the ore is heated in the phosphorus furnace. The fluorine probably is discharged from the furnace as silicon tetrafluoride (SiF.sub.4). PA0 3. Air enters the furnace through feed chutes when materials are fed into the furnace. Also, air enters the furnace through openings in the furnace roof. Oxygen in the air oxidizes the elemental phosphorus to P.sub.2 O.sub.5 and the P.sub.2 O.sub.5 mixes with the furnace gas stream. Nitrogen also mixes with the furnace gas. PA0 4. Small particles of furnace feed materials become suspended in the gas. PA0 5. Metals and metal compounds are volatilized by the electric arc in the furnace. These materials condense as small particles which become suspended in the furnace gas stream. PA0 1. Complete removal of elemental phosphorus from the waste will not be necessary. PA0 2. Constituents in the solid wastes have value as furnace feed. PA0 3. Toxic metals in the wastes become encapsulated in the glassy slag, or these metals alloy with ferrophosphorus. In both cases the toxic metals are rendered innocuous and they are ultimately disposed of as byproducts. PA0 4. Phosphorus-containing wastes can be converted into useable products at the rate they are generated. Accumulation of the toxic wastes at the plant site can be eliminated. PA0 1. Phosphorus furnace plants producing high-quality phosphoric acid. PA0 2. Phosphorus furnace plants producing high-quality elemental phosphorus. PA0 3. Plant sites where neither phosphoric acid nor elemental phosphorus is produced. PA0 1. Burning phosphorus sludge to make a mixture of phosphorus oxides. PA0 2. Reacting the mixture of phosphorus oxides with water to make an acidic mixture. PA0 3. Recycling acidic mixture prepared in 2 to low-temperature agglomerating processes wherein phosphate ore is prepared for smelting. PA0 4. Feeding agglomerated phosphate ore prepared in 3 to a submerged-arc electric furnace. PA0 5. Smelting agglomerated phosphate ore to produce phosphorus and phosphorus sludge. PA0 1. Agglomerate phosphate ore by low-temperature process wherein B.I. material is incorporated in agglomerates. PA0 2. Smelt agglomerates prepared in 1 in a submerged-arc electric furnace. PA0 3. Incorporate condenser bleedoff water into a process for the production of orthophosphate suspension fertilizer. PA0 4. Pump phosphorus and phosphorus sludge to a tank wherein phosphorus is separated from phosphorus sludge by a gravity-separation process. PA0 5. Fluidize phosphorus sludge with an alkaline dispersing agent. PA0 6. Evaporate phosphorus sludge in a still to recover high-quality phosphorus in a condensing assembly. PA0 7. Recycle residue from still to agglomerator in step 1. PA0 8. Add phosphorus recovered in still to high-quality phosphorus storage.
Furnace gases are treated to condense the elemental phosphorus from the gas mixture, thereby separating the phosphorus from the other gases. FIG. 1 in patent application Ser. No. 503,099 illustrates the process for treating phosphorus furnace gas stream. The gases are treated in an electrostatic precipitator to remove particulates. A small amount of elemental phosphorus is collected with the particulates. The gases are then contacted with water to condense phosphorus by adiabatic cooling. The gases are further cooled by water in a tubular cooler to condense additional phosphorus. Then the gases are exhausted by a compressor.
The composition of the condenser exhaust gas depends largely on the type and composition of the reducing carbon used in the furnace. Metallurgical coke is frequently used as a reducing carbon. The analyses shown in Table 1 were obtained when metallurgical coke was being used.
TABLE 1 ______________________________________ Composition of Condenser Exhaust Gas.sup.a Percent by volume Constituent on a dry basis ______________________________________ CO.sub.2 0.7 O.sub.2 0.1 CO 87.3 H.sub.2 7.9 N.sub.2 2.3 Unsaturated 0.6 hydrocarbons.sup.b CH.sub.4 1.1 ______________________________________ .sup.a Metallurgical coke was the reducing carbon. .sup.b Assumed to be ethylene.
The gross heating value of the gas was calculated to be 343 Btu per cubic foot at STP. About 80,500 cubic feet (STP) of gas was obtained per ton of phosphorus produced. The potential energy in the gas was about 27.6 million Btu per ton of phosphorus produced. Energy in the metallurgical coke was about 38.2 million Btu per ton of phosphorus, and the condenser exhaust gas has about 72 percent as much energy as the reducing carbon.
The furnace gas contains about 24.9 pounds of elemental phosphorus per 1,000 cubic feet of noncondensable gas at STP, and this is equivalent to about 6 percent phosphorus by volume. Dust is collected in the precipitator at a rate of about 0.07 ton per ton of elemental phosphorus produced. The gas mixture is essentially dry, but it is cooled from about 600.degree. F. to about 145.degree. F. by saturating with water vapor in a spray chamber. Phosphorus content of the gas is reduced to about 0.3 pound per 1,000 cubic feet of noncondensable gas at STP by cooling in the spray chamber. About 98.8 percent of the phosphorus in the furnace gas is recovered by adiabatic cooling.
The gases are further cooled in a condenser wherein gases flow through the tubes and cooling water is outside. Water is sprayed into the tubes to irrigate the inside surfaces and prevent fouling. The gases are cooled to about 128.degree. F. in the tubular cooler and the phosphorus content of the gases is reduced to about 0.2 pound per 1,000 cubic feet of noncondensable gas at STP, resulting in an overall phosphorus recovery of about 99.2 percent. Water is condensed in the tubular cooler at a rate of 484 pounds per ton of phosphorus produced. After condensation of the phosphorus the gases are exhausted by a Nash Hytor pump.
Both the adiabatic condenser and the tubular cooler drain to a sump as shown in FIG. 1, application Ser. No. 503,099. Condensate from the condensers is a mixture of water, liquid phosphorus, and an impure phosphorus product called phosphorus sludge. The sump is designed to separate the three materials as shown in FIG. 91 in Chemical Engineering Report No. 3. Twoopartitions are in the sump, thereby dividing it into three compartments. Liquid phosphorus has a specific gravity of 1.73 at 140.degree. F. and it separates from other materials by collecting in the compartment under the condenser drain. The liquid phosphorus is periodically pumped to storage tanks by submerged pumps. Water and phosphorus sludge overflow a partition and flow into the middle compartment and phosphorus sludge collects in the bottom of this compartment because it has a specific gravity of about 1.23. The phosphorus sludge is periodically removed from the compartment by submerged pumps and is stored in tanks. Water overflows the second partition and enters the third compartment called the spray liquor compartment. From this compartment the water is recirculated to the Nash Hytor pump, the tubular cooler, and the adiabatic condenser by pumps.
The electrostatic precipitators at phosphorus furnaces are provided to collect solid particles that become entrained in the gas stream. From 60 to 90 percent of the entrained solids are collected. Uncollected solids are removed from the gas stream by contacting the gases with water sprays in the adiabatic condenser and the tubular cooler.
Phosphorus sludge becomes viscous after storage. Also, the material may be sticky depending on the type of reducing carbon used. Microscopic examination of the sludge shows that it consists of globular particles of yellow phosphorus as large as 1 to 2 millimeters in diameter and the particles may be only a few microns in diameter. Some of the smaller globules agglomerate into clusters. Electrical charges and physical barriers of foreign solids are thought to retard or prevent coalescence of the phosphorus particles.
Following is the approximate composition of the major solid constituents in the sludge.
P.sub.2 O.sub.5 : 32 percent PA1 F: 23 percent PA1 CaO: 8 percent PA1 SiO.sub.2 : 7 percent
The proportion of total phosphorus produced as phosphorus sludge varies widely, but data given in Chemical Engineering Report No. 3, Tables X and XII, may be used to determine a typical proportion, as indicated below.
______________________________________ Tons elemental phosphorus per day ______________________________________ 22.4 tons per day of phosphorus 21.91 containing 97.8 percent phosphorus = 5.5 tons per day of phosphorus sludge 3.79 containing 68.9 percent phosphorus = Total 25.70 ______________________________________
The data above indicate 14.7 percent of the phosphorus is collected as phosphorus sludge when separation is made in the condenser sump. The phosphorus content was 68.9 percent and this was rich enough to burn in a phosphoric acid production unit.
When phosphorus sludge is stored, heated, or agitated, some of the particles of yellow phosphorus coalesce. When the material is stored in a tank, liquid phosphorus will collect in the bottom of the tank. The phosphorus can be pumped out and the yield of good phosphorus is thereby increased. When this occurs the percentages of solids and water increase. Upon prolonged storage with occasional melting and agitation, the phosphorus content of the sludge may be reduced to the range of 5 to 15 percent. The material becomes very viscous and the stickiness increases. Material with depleted phosphorus content is called consolidated sludge, and it possesses characteristics which prevent it from being pumped.
In addition to storing, heating, and agitating, the recovery of good phosphorus can be increased by filtering or centrifuging the phosphorus sludge. In both cases residues are obtained and the residues contain too much phosphorus to be discarded. When liquid or solid materials contain elemental phosphorus, serious pollution problems arise when the materials are stored or discharged as wastes. Furthermore, filtering or centrifuging phosphorus sludge is both hazardous and costly.
The phosphorus in phosphorus sludge can be evaporated by heating the material in a still. A carrier gas can be provided to sweep out the phosphorus in the still and the phosphorus can be separated from the carrier gas by condensing. The gas mixture can be cooled by contacting it with water by an arrangement similar to that shown in FIG. 1, application Ser. No. 503,099. A still is shown as FIG. 4 in the present application. Prolonged heating of sludge in the still will evaporate essentially all the phosphorus from the solids, but a large amount of fuel is required.
At the Tennessee Valley Authority National Fertilizer Development Center, phosphorus sludge was burned and P.sub.2 O.sub.5 was formed. The P.sub.2 O.sub.5 was reacted with water to produce phosphoric acid in accordance with a process described in Corrosion, Volume 14, 21-6, August 1958. Operation of the phosphoric acid unit with various compositions of phosphorus sludge disclosed that the minimum phosphorus content required for combustion was about 60 percent. Freshly made sludge normally contained enough phosphorus to burn, as discussed above, but consolidated sludge or residues obtained by filtering or centrifuging were too lean to burn. Dust is discharged from precipitators as a slurry and the slurry contains elemental phosphorus. However, the phosphorus content of the precipitator dust is too low for the material to be burned to produce phosphoric acid. When the sludge contained less than about 60 percent phosphorus the material had to be enriched with phosphorus to increase its calorific value. Treatment of sludge to increase the recovery of good phosphorus was of little benefit.
The phosphoric acid produced by burning phosphorus sludge is black and it contains solid impurities. Only orthophosphoric acid could be produced because of the relatively low calorific value of the sludge. The phosphoric acid was used for the production of fertilizers, and non-orthophosphoric acid was preferred. Phosphorus in sludge was not completely oxidized to P.sub.2 O.sub.5. Some of the phosphorus was oxidized to P.sub.2 O.sub.3 and a mixture of phosphoric and phosphorous acids was formed. The value of phosphoric acid made from sludge was less than that made from good phosphorus. Consequently, the alternative of burning phosphorus sludge to produce impure phosphoric acid is not attractive.
At times during World War II, all the phosphorus produced at the TVA National Fertilizer Development Center was used in munitions. None of the phosphorus was converted into phosphoric acid. During these periods the phosphorus and phosphorus sludge were agitated and elutriated with hot water in equipment called a washer. High-quality phosphorus was produced as required for munitions. Solid impurities and phosphorus droplets overflowed the washer. This watery mixture was discharged into a 14-acre settling pond to clarify the water before it was discharged into a watercourse.
Clarified water contained dissolved elemental phosphorus and colloidal phosphorus particles in suspension. The clarified water was toxic to marine animals, and fish were unable to survive in the receiving stream. Breakouts occurred at the settling pond, resulting in the release of additional hazardous waste to the watercourse. Phosphorus production was discontinued at the TVA National Fertilizer Development Center in 1976. Nevertheless, the settling pond continues to be a potential pollution hazard. Additional breakouts of phosphorus-containing solids may occur and water in the settling pond may be inadvertently discharged to the receiving stream. Heretofore, technology has not been available to dispose of the hazardous wastes.